Screen Resolution Calculator

What PPI measures

PPI stands for pixels per inch, how many pixels are packed into every inch of the screen. It is the number that tells you whether an image or a line of text will look crisp or blocky, and it depends on two things: the resolution and the physical size of the panel.

The formula is straightforward. Count the pixels along the diagonal (the square root of width squared plus height squared), then divide by the diagonal measurement in inches:

PPI = sqrt(width_px² + height_px²) / diagonal_inches

Resolution alone tells you nothing about sharpness. A 1920 x 1080 panel is 92 PPI on a 24-inch monitor, 82 PPI on a 27-inch, and 141 PPI on a 15.6-inch laptop. Same number of pixels, three different experiences. The World Wide Web Consortium even defines the CSS pixel as a reference unit independent of physical pixels, precisely because device densities vary so much.

The other half of the story is viewing distance. A 60-inch 4K television has only 73 PPI, yet it looks perfectly sharp from a couch ten feet away, because your eye cannot resolve individual pixels at that distance. The calculator reports both: PPI, and pixels per degree of view at your seating distance.

Worked example: 1080p on a 24-inch monitor

The most common desktop setup in the country. Here is the full working.

• Width squared: 1920 x 1920 = 3,686,400
• Height squared: 1080 x 1080 = 1,166,400
• Sum: 4,852,800
• Square root: 2,202.9 pixels on the diagonal
• Divide by 24 inches: 91.8 PPI
• Aspect ratio: 1920 / 1080 reduces to 16:9
• Total pixels: 2,073,600, or 2.07 megapixels
• Dot pitch: 25.4 / 91.8 = 0.277 mm

At 0.277 mm, most people can see the pixel grid if they lean in, and text edges show slight fringing. It is not bad, it is just ordinary. Now move the same 1080p panel to a 15.6-inch laptop: the diagonal is the same 2,202.9 pixels but divided by 15.6 inches, giving 141 PPI and a dot pitch of 0.180 mm. That is a sharpness most desktop users have never seen, and it is why laptop screens often look better than bigger monitors despite the same resolution.

Worked example: 1440p on a 15.6-inch laptop

Now a high-density case: 2560 x 1440 on a 15.6-inch panel.

• Width squared: 2560 x 2560 = 6,553,600
• Height squared: 1440 x 1440 = 2,073,600
• Sum: 8,627,200
• Square root: 2,937.2 pixels on the diagonal
• Divide by 15.6 inches: 188.3 PPI
• Aspect ratio: 16:9
• Total pixels: 3,686,400, or 3.69 megapixels
• Dot pitch: 0.135 mm

At 188 PPI and a 0.135 mm dot pitch, individual pixels are not visible at any normal distance. The screen needs 3.69 million pixels driven instead of 2.07 million, though, which is 78% more work for the graphics processor, the reason high-density laptops get warm and battery life drops.

One practical consequence: at this density the operating system usually applies 150% or 200% display scaling, so everything is not tiny. That is worth knowing because scaling changes how much you can fit on screen. A 1440p laptop at 200% scaling gives you the usable area of a 1280 x 720 desktop, fewer windows, not more.

Common resolutions and their pixel density

The same resolution looks different at different screen sizes. Here is how the mainstream options compare.

Read down the columns rather than across. 1080p is 92 PPI on a 24-inch monitor and 141 on a 15.6-inch laptop, a 54% increase in density from size alone. 4K on a 27-inch screen is 163 PPI, and 4K on a 24-inch is 184 PPI, which is past the point where anyone can see the difference at desk distance.

The International Organization for Standardization and the National Institute of Standards and Technology both maintain measurement standards for displays and digital imaging, which is where the industry definitions ultimately come from. In practice, the useful summary is that anything above about 150 PPI at desk distance is beyond what the eye resolves.

Aspect ratio: the number most people ignore

Aspect ratio is the shape of the screen, width divided by height. Almost every modern display is 16:9, but the exceptions matter, and they change what you can do with the space.

The calculator reduces your resolution to its simplest ratio, so a 3440 x 1440 panel comes out as 21:9 rather than 1720:720. That matters when you are buying: a 34-inch 21:9 ultrawide has roughly the same screen height as a 27-inch 16:9 monitor, so it is not as tall as the diagonal measurement suggests.

Does PPI matter at your viewing distance?

Sharpness is a relationship between pixel density and how close you sit, not an absolute property of the screen. The useful measure is pixels per degree, how many pixels fall within one degree of your field of view. The human eye resolves roughly 60 pixels per degree at the centre of vision, which is the number behind the whole "retina" idea.

The formula is simple: pixels per degree = PPI x viewing distance x 0.01745. A 24-inch 1080p monitor at 92 PPI viewed from 24 inches gives 92 x 24 x 0.01745 = 38 pixels per degree. That is well short of 60, which is exactly why you can see the pixels if you look. The same monitor at 36 inches gives 58 pixels per degree, effectively retina, because you moved back.

This is why a 4K phone and a 4K television can both look perfect despite wildly different PPI: the phone is 12 inches from your face and the TV is 120. It also explains why buying a higher-resolution monitor and then pushing it further away on the desk can cancel out the benefit. If you want sharper text, move the screen closer before you buy a new one, it is free.

Resolution names: HD, Full HD, QHD and 4K

The names are marketing shorthand for the vertical pixel count, and they are used loosely. Here is what they mean.

HD (720p) is 1280 x 720-0.92 megapixels. It is now rare outside cheap laptops and small TVs. HD (768p) is 1366 x 768, the resolution of millions of budget laptops, and its aspect ratio is only approximately 16:9 (1366 / 768 = 1.7786 against the true 1.7778).

Full HD (1080p) is 1920 x 1080-2.07 megapixels, and still the most common desktop resolution. QHD (1440p) is 2560 x 1440-3.69 megapixels, sometimes called 2K, which is a misnomer. 4K UHD is 3840 x 2160-8.29 megapixels, exactly four times 1080p, which is where the name comes from. Cinema 4K is 4096 x 2160, slightly wider, and you will only meet it on professional gear.

One trap: a "4K" television and a "4K" monitor have the same resolution but different pixel densities, because the TV is far bigger. A 55-inch 4K TV is 80 PPI; a 27-inch 4K monitor is 163 PPI. Both are 4K, and they do not look remotely alike up close.